The need to turn to coal as a principal energy source has provided an impetus for examining various methods of burning the fuel in an environmentally acceptable manner. Among the methods in which interest has been rekindled is that of burning the coal in a fluidized bed. In a fluidized-bed arrangement, coal and air are reacted in a bed of particulate matter that is agitated by the flow of the air to the extent that it attains a quasi-liquid state. The advantages of this mode of burning coal lie in the ability of the bed to burn the coal in a comparatively small volume, to conduct heat relatively rapidly to heating surfaces placed in the bed, and to absorb the sulfur in the coal if the fluidized medium includes material that reacts with the oxidized sulfur.
The relatively rapid conduction of the heat to the heating surfaces results from the high thermal conductivity that characterizes the quasi-liquid mass of particles in the bed. Unfortunately, the high conductivity of the bed in the fluidized state makes stable operation at low firing rates difficult. A fluidized bed that liberates 1.times.10.sup.6 Btu/Hr--and which therefore has associated with it heat-conduction surfaces that absorb heat at that rate--may only have 4.times.10.sup.4 Btu associated with it at a temperature of 1500.degree. F. (820.degree. C.). Consequently, a small imbalance between the rate of heat liberation and the rate of heat removal can cause the bed temperature to fall by a relatively large amount. Such an imbalance, caused, for instance, by a momentary reduction in fuel-supply rate, can use the bed temperature to fall below the ignition temperature of the fuel, particularly when the average firing rate, and thus the bed temperature, is already relatively low. Since the ignition of fuel in a fluidized-bed boiler is dependent predominantly upon bed temperature, the almost unavoidable heat-flow imbalances in the system can cause the bed to be extinguished at low loads.